1. Field of the Invention
This invention relates to roofing compositions having algicidal properties. More particularly, it relates to roofing granules and to roofing shingles incorporating said granules and possessing algicidal properties for the effective retardation of the biological growth of algae and/or fungi on roofing surfaces.
2. Description of the Prior Art
Roofing granules, both natural and artificially color-coated granules, are extensively used in roll roofing and asphalt shingle compositions. The roofing granules are generally embedded in the asphalt coating on the surface of an asphalt-impregnated felt base material, the granules thus forming a coating that provides an adherent, weather-resistant exterior roofing surface. As this outer granule coating also provides the esthetic effect observable with respect to the roofing composition, the appearance of the granules is of major marketing interest. For this reason, therefore, a pigmented color coat is ordinarily applied to the base mineral granules to enhance their visual, decorative effect.
As white or light-colored roofs are particularly favored in warmer climates, TiO.sub.2 pigment is commonly used in the production of light color-coated roofing granules. In such warmer climates, as in the southern part of the United States, discoloration of asphalt roofing compositions by the growth of algae and/or fungi is of particular concern. Such discoloration, of course, is particularly noticeable on the white or light-colored roofs otherwise so desired and popular in such regions. Upon discoloration, the roof becomes unsightly in appearance and is found to cause a greater heat absorbence as, for example, when a white roof is turned dark brown or black in a period of a few years in use.
Nor is this problem, so widespread in areas such as the southern U.S. particularly the gulf state area, confined necessarily to such regions. Thus, discoloration of roofing surfaces by the growth of algae and/or fungi has also been found in the northern part of U.S., particularly so in areas along rivers and lakes and along the northern coastal regions. While home owners and others have been aware of this discoloration problem for many years, effective, practical solutions thereto have not been forthcoming at a reasonable cost. The problem of roofing granule discoloration, therefore, has remained a major marketing problem for the roofing industry.
For many years, this problem of roofing granule discoloration was believed to be caused only by fungi, as is the case with respect to some outdoor paint surfaces. Many different types of fungi have, in fact, been isolated from discolored roofing surfaces. More recently, however, it has been learned that other organisms contribute principally to this discoloration and have been identified as terrestrial blue-green algae of the Cyanophyta division. Such algae are transferred through the air as spores and/or vegetative matter and deposited on roofing surfaces where they thrive and grow. Natural pigments produced by the algae add to the dark discoloration of the roof, which is generally first noticeable in spots that develop into dark vertical streaks that gradually darken until the entire roof becomes a totally discolored black within five to fifteen years. Predominant algae thus identified from infested roofing shingles include Gloeocapsa Magma, Tolypothrix Byssoidea, Nostoc sp. and Scytonema sp. In general, metallic algicides that are effective in retarding the biological growth of such algae are likewise effective in similarly retarding the growth of fungi. The incorporation of a metallic algicide in the color coat of roofing granules, therefore, has heretofore been proposed in order to inhibit or prevent the discoloration of roofing surfaces containing such granules as a result of algae and/or fungi growth.
The incorporation of a metallic copper algicide in the color coat of roofing granules was disclosed in the Skadulis patent, U.S. Pat. No. 3,528,842. Skadulis particularly proposes the incorporation of copper algicides that are substantially waterinsoluble but that have limited solubility in acidic solutions, e.g., Cu.sub.2 O. Similarly, slightly soluble zinc algicides were disclosed for incorporation in the color coat of roofing granules in the McMahon patent, U.S. Pat. No. 3,507,676. As was pointed out in the McMahon patent in Column 2, lines 58-65, such zinc algicides are effective when employed in an amount constituting at least about 1% by weight of the base mineral granules, i.e., about 20 lbs. of the zinc algicide compound or metal per ton of granules. The incorporation of particular copper or zinc algicides in the color coat of roofing granules, in the manner and in the quantities taught by the Skadulis and McMahon patents, imparts a desirable resistance of roofing surfaces containing such granules to discoloration upon exposure to atmospheric weathering. The teachings of these patents, however, have not led to the development, commercial availability and use of algicidal roofing granules providing the desired degree of algae and/or fungi resistance over an extended period of time at economically competitive cost. In part, of course, this unavailability of a totally satisfactory algicidal roofing granule reflects the continual desire in the roofing industry for a more effective algicidal effect from a roofing granule of ever diminishing incremental cost to achieve such an algicidal effect. Any incorporation of metallic algicides in an otherwise conventional roofing granule coating in order to achieve the necessary or desirable algicidal effect necessarily adds an incremental cost to the roofing granule and to the roofing material incorporating such an algicidal granule. While the desired toxic effect is a necessary or highly desirable feature of the algicidal roofing granule, the providing of this property or function is an expense item that, from a marketing viewpoint, must be minimized to the fullest possible extent. The use of minimum quantities of metallic algicides to produce a desired level of effectiveness over an extended period of time is, therefore, highly desirable. While the prior techniques have imparted an algicidal effect to roofing granules, an enhanced effect would provide further assurance of the desired toxic effect, thereby enhancing the quality of such granules. In this regard, it should be noted that the algicidal granules of McMahon require the incorporation of a relatively large amount of zinc for effective algicidal action as noted above. As the amount of metallic algicide required for effective action increases, the cost of the resulting algicidal granule is directly increased thereby. In addition, the use of relatively large amounts of metallic algicides frequently requires the incorporation of pigment in the granule coating in amounts in excess of that otherwise required to achieve a desired roofing granule color. As the amount of Cu.sub.2 O employed is increased, for example, the amount of TiO.sub.2 pigment that must be employed in the granule coating composition to produce a white roofing granule is also generally increased. Such an additional requirement necessarily adds to the overall cost of the algicidal roofing granule product and of roofing materials made therefrom.
The prior art above is, of course, subject to the practical necessity of producing such algicidal granules as a separate production operation apart from the production of conventional, non-algicidal, color-coated roofing granules because of the variation in the color coating formulation necessarily required to incorporate the algicidal compound into the color coating. The interruption of such conventional roofing granule production and the related scheduling and inventory problems associated therewith tend to create a further economic disadvantage associated with the production and marketing of algicidal roofing granules and roofing compositions incorporating said granules.
In another prior art approach, conventional, non-algicidal granules are employed, and relatively coarse granular zinc or zinc-containing material is blended with said granules prior to the embedding thereof in the asphalt coating of the roofing shingle composition. This approach, disclosed in the Klimboff patent, U.S. Pat. No. 3,598,627, results in the production of an asphalt-impregnated base that contains embedded, on all or a portion of its exterior surface, said granular zinc or a granular zinc-containing material along with the conventional colored granules. The granular zinc is disclosed as having a particle size distribution within the overall range of about -10+35 mesh and an effective dosage level of from about 3% to about 10% by weight based on the weight of conventional, colored granules employed. While this approach thus overcomes some of the production, scheduling and inventory problems associated with the incorporation of an algicidal agent in the color coat itself, it will be seen that the relatively high amounts of zinc that must be employed and the nature of the distrubution of the relatively large zinc particles in the asphalt coating along with said conventional granules necessarily produces an incremental cost-algicidal effectiveness relationship not fully satisfying the overall commercial imperatives relating to the marketing of roofing granules and roofing composition, as discussed above. There is a need in the art, therefore, for improved algicidal products and techniques so that effective inhibition or prevention of the discoloration of roofing surfaces due to the growth of algae and/or fungi can be provided in the art with an economically feasible balance between the desired algicidal effectiveness and the incremental cost to achieve such algicidal properties. Such a need must also be satisfied, as indicated above, in a manner permitting optimum flexibility and compatibility with the ordinary requirements of the conventional production of non-algicidal roofing granules and compositions.
It is an object of the present invention, therefore, to provide improved algicidal roofing granules and compositions.
It is another object of the invention to provide an improved process for the production of such algicidal roofing granules and roofing compositions incorporating same.
It is another object of the invention to provide algicidal roofing granules and compositions having effective, long-term algicidal properties at economically desirable, low levels of metallic algicide content.
It is another object of the present invention to provide a process for the production of algicidal roofing granules and roofing compositions incorporating said granules with enhanced operating flexibility and compatibility with the requirements of conventional granule and roofing production operations.
It is a further object of the invention to provide roofing surfaces having an enhanced resistance to discoloration during extended periods of exposure to atmospheric weathering.
With these and other objects in mind, the present invention is hereinafter set forth in detail, the novel features thereof being particularly pointed out in the appended claims.